An experimental vaccine to prevent cervical cancer protected virtually all the women who took it during a large international trial, boosting chances that future generations of girls throughout the world might live their lives free of risk of the disease.

Results of the two-year study, involving more than 12,000 women, were released Thursday at a conference of infectious disease specialists convened this week in San Francisco.

"This is certainly very good news for women," said Dr. Jay Lalezari, a UC San Francisco physician who conducted tests of the vaccine among Bay Area volunteers during the past four years, including the most recent two-year trial.

The new vaccine prevents cervical cancer by blocking certain strains of the human papilloma virus. Chronic infection with HPV -- a sexually transmitted virus -- is responsible not only for cervical cancers, but also precancerous lesions and genital warts. It is also implicated in anal cancers among gay men.

Developed by New Jersey pharmaceutical giant Merck & Co., the vaccine targets two strains of HPV that are known to cause at least 70 percent of cervical cancers, and two other strains that cause genital warts.

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Cervical cancer is relatively uncommon in the United States, due to a vigorous screening program of annual Pap smears designed to detect early signs of the disease. Still, it strikes more than 10,000 women annually, and each year it kills about 3,700 in the United States, according to the American Cancer Society.

Worldwide, it is the second-leading form of cancer among women, and the third-leading cause of women's cancer deaths -- ranked behind lung and breast cancer.

Each year, cervical cancer kills 290,000 women worldwide, most of them in poor countries.

The trial, which took place from 2002 to 2004, suggests that the vaccine is dramatically effective in preventing the early stages of cancer caused by the HPV infection. In a group of 12,167 women from 13 countries, half received the vaccine and half a placebo. The women were given three shots: an initial vaccination, a second dose two months later, and a final booster six months later.

Among the vaccinated women, there were no cases of precancerous lesions or cancer after two years; there were 21 cases observed in women who received the placebo. A second analysis that included women who did not adhere rigidly to their scheduled appointment for inoculations found only one positive test in the vaccinated group.

Results of a second large-scale trial to determine the effectiveness of the vaccine against genital warts are still pending.

Based on the highly favorable cervical cancer findings, Merck & Co. plans before the year's end to seek approval from the Food and Drug Administration to market the new vaccine.

"You almost never see 100 percent efficacy for any vaccine," said Lalezari. The San Francisco physician, who is also a prominent AIDS doctor, said he is hopeful the vaccine will be effective in preventing HPV infection in gay men.

"I lose more HIV patients to anal cancer than anything else," he said. "It is the one opportunistic infection that has not gotten better in the era of HAART," the three-drug combinations that keep the AIDS virus in check.

Dr. Eliav Barr, the Merck scientist who directed the testing program, noted that the vaccine was tested among women who were not yet infected with HPV. "It will probably not help those who are already infected," he said. "The kind of immune response needed to clear HPV is different from the one to prevent it."

That means that the long-term value of the vaccine may be in protecting young women -- and possibly young men -- from infection before they become sexually active.

"This vaccine will have to be promoted by pediatricians, as well as by OB/GYNs," said Dr. Michael Randell, an obstetrician and gynecologist who lectures to other physicians on HPV detection.

Doctors involved in cervical cancer prevention acknowledge that prevention of the disease can be a delicate topic, because HPV is a sexually transmitted disease. It can be transmitted with simple skin-to-skin contact -- rather than by an exchange of body fluids -- so condoms are not deemed to be protective against it. As a consequence, HPV is the most common sexually transmitted disease in the United States -- an estimated 20 million men and women are infected.

Erin Schwartz, 24, signed up for the vaccine trial as a college student in Iowa. She said she didn't know at first about the connection between HPV and cervical cancer, but she knew the virus could also cause warts.

Schwartz worked for a university office that evaluated human drug testing, and so she was aware of experiments under way and signed up partly to make some extra cash, she said.

She was paid $100 for each visit for a vaccination, evaluation or blood test, and she figures she has made $800 as a human guinea pig on the study. She said she still does not know whether she got the real shot or the placebo.

"I've had normal Pap smears the whole time, but I sure would like to know whether I had the real vaccine," she said.

Most people are able to clear the virus through natural immune processes, but women with chronic infections run a risk of developing cervical cancer, as gay men are at risk for anal cancer.

Because the vaccine prevents the strains of HPV blamed for only 70 percent of cervical cancers, women will still need to have regular Pap smears to screen for the disease. Doctors are hopeful that the vaccine will in fact provide some protection against the remaining strains of the virus, but that has yet to be proven.

The most promising benefit of a cervical cancer vaccine may be in the developing world, where routine Pap smears don't occur and cervical cancer is the leading cause of cancer death among women.

Merck's Barr declined to discuss pricing of the vaccine. It is made of genetically engineered bits of the outer protein coats of four papilloma virus strains -- a process that has yielded safe, effective, but costly vaccines such as that against hepatitis B.

However, Barr said his company is working closely with the Bill & Melinda Gates Foundation to find ways of bringing the vaccine to poor countries. "We are working very closely with governments and nongovernment organizations to implement a strategy for the developing world," he said.

Researchers hope the vaccine will provide lifetime immunity from HPV, but the only way to prove that will be with long-term follow-up. For that reason, many of the volunteers in the study were recruited from Scandinavian countries, where medical records are tracked for a lifetime. "We will follow the women in this trial for a very long period of time," Barr said.

Facts about cervical cancer

A genetically engineered vaccine had a 100 percent rate of preventing the most common forms of cervical cancer in a two-year study. The vaccine blocks infection from certain strains of the human papilloma virus that are the leading cause of cervical cancer.

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